The present invention relates to a method for detecting considerably small particles of foreign matter on a sample and a device for realizing the same and in particular to a method for detecting considerably small particles of foreign matter, which is suitable for detecting foreign matter on a wafer, which is a (patterned) product, and a device for realizing the same. The present invention can be applied to samples such as semiconductor LSI wafers, glass masks, the surface of magnetic disks, etc.
With the rapid progress of the semiconductor technology the pattern of semiconductor devices has been made finer from the conventional 5 .mu.m level to the 1 .mu.m level. In keeping therewith, in the semiconductor process, a method for detecting considerably small particles of foreign matter corresponding to the 1 .mu.m level and a device for realizing the same have been necessitated.
In view of the circumstances described above the present invention has been done in order to solve the problems of the prior art method and device for detecting foreign matter.
Hereinbelow the prior art technique will be explained.
Taking detection of foreign matter on a patterned wafer as an example, according to the prior art technique, e.g. as represented by that disclosed in JP-A-54-57126, the circuit pattern on the wafer and the foreign matter thereon are irradiated directly with polarized light and attention is paid to differencies in the degree of depolarization of the light reflected by each of them. That is, the wafer 1 is irradiated obliquely with S polarized light beams emitted by laser devices 70a and 70b, as indicated in FIG. 5. An "S.infin. polarization component, or "Senkrechte" in German, which translates to "vertical or perpendicular line" in English, represents a light component which oscillates in a direction perpendicular to the direction of the plane formed by the illumination light axis. The "P" polarization component, or "Parallele" in German, which translate to "parallel line" in English, represents a light component which oscillates in a direction parallel to the direction of the plane formed by the illumination light axis and perpendicular to the "S" polarization component. In general, since the circuit pattern 71 on the wafer is constructed usually by regular straight line stepwise patterns, the depolarization of the laser light is slight and the S polarization component is conserved in the light 74 reflected by straight line edges in the pattern 71, which are perpendicular to the optical axis of the laser beam 103 as it is. On the other hand, since the shape of the foreign matter is irregular and it can be thought that it is composed of infinitesimal surfaces having various incident angles for the incident laser light, the laser light is scattered. As the result, the scattered light is depolarized and there exist mixedly S and P polarization components in the scattered light 75. Therefore, if a polarizing plate 76 is disposed above the objective lens 7 so as to cut-off the S polarization component (indicated by a full line), only the P polarization component in the light 75 scattered by the foreign matter is detected by a photo-electric converting element 77, as indicated by 79.
FIGS. 6A and 6B show the polarization state of the light scattered by the foreign matter before and after the passage through the polarizing plate respectively, according to the prior art technique. As it is clearly seen from the figures, according to the prior art technique, the P polarization component, which can pass through the polarizing plate, is a considerably small part of the whole light scattered by the foreign matter and the lower limit of the size of detectable particles of foreign matter is about 3 to 5 .mu.m. That is, according to the prior art technique, the polarizing plate is used for removing reflected light coming from the pattern on the sample and this results in that the great part of the light scattered by the foreign matter is also removed. Consequently as indicated in FIG. 7, in the case where the size of the particles of foreign matter 84 is further smaller and it is 1 to 12 .mu.m, it is very difficult to detect them because of decrease in light quantity of the whole scattered light itself and decrease in the light quantity due to the polarizing plate. If the laser light intensity were increased in order to increase the detected light quantity, light scattered by pattern corner portions, which are otherwise not so strongly brilliant, would pass through the polarizing plate, which makes it difficult to discern the foreign matter from the other. Further there are particles of foreign matter, whose depolarization is small, depending on the matter and the shape thereof. In this case almost no P polarization component is contained in the light scattered by the foreign matter which makes it still more difficult to detect them.
An object of the present invention is, in view of the problem of the prior art technique described above, to provide a method for detecting foreign matter and a device for realizing the same capable of detecting light scattered by the foreign matter with a high efficiency independently of the depolarization.
Further there is known another prior art device for detecting foreign matter, as disclosed in an article entitled "A Laser Scan Technique for Electronic Materials Surface Evaluation" by D. R. Oswald J. of Electronics Materials, Vol. 3, No. 1, January 1974, etc.
FIGS. 32 to 34 indicate the prior art principle for detecting foreign matter.
A downward illuminating optical system B consists of a laser light source 501, a focusing lens 502, a polarizing prism 503, a field lens 504, a 1/4-wave plate 505 and an objective lens 506.
On the other hand a detecting optical system consists of a light intercepting plate 508, an imaging lens 509 and a detector 510.
A laser light beam 511 outputted by the laser light source 501 is S-polarized. It passes through the polarizing prism 503 and forms a laser spot 511a in a diaphragm 504a in the field lens 504. The laser light beam 511 passing through the field lens 504 passes through the 1/4-wave plate 505 and forms a laser spot 511c on the sample 1 owing to the objective lens 506.
In the case where there is no foreign matter on the sample 1, laser light reflected by the surface of the sample (0-th order diffraction light) 511 passes again through the objective lens 506, the 1/4-wave plate 505 and the field lens 504. Then it is intercepted by the light intercepting portion 508a of the light intercepting plate after having been reflected totally by the polarizing prism 503. Here the field lens 504 images the extension 511b of the laser light beam 511 at the diaphragm 506a to project it on the light intercepting portion 508a,. The light intercepting portion 508a in the light intercepting plate 508 is obtained by forming an opaque film at the central portion on a transparent glass.
In this case, since S polarized light in the illuminating light 511 is changed into P polarized light in the reflected light 511, when the illuminating laser light 511 passes through the 1/4-wave plate 505, and further the reflected laser light 511 passes therethrough, the reflected light 511 is reflected totally by the polarizing prism 503.
In the case where there exists foreign matter 513 on the sample, when the foreign matter 513 is irradiated with the illuminating light 511, light 512 scattered by the foreign matter (high order diffraction light) is produced, which is spread over the whole area of the diaphragm 506a in the objective lens 506 and returns along the same optical path as that of the reflected light 511 described previously.
The surface of the foreign matter 513 presents considerably fine uneven shapes and the scattered light 512 is depolarized, having both S and P.
The P polarized light 512b in the scattered light 512 passes through the transparent portion outside of the light intercepting portion 508a in the light intercepting plate 508, after having been reflected by the polarizing prism 503, and is collected by the imaging lens 509 to be led to the detector 510.
The prior art technique described above had problematical points (1) to (3) as follows:
(1) The S polarized light 512a returns to the laser light source 501 after having passed through the polarizing prism 503 and the focusing lens 502 so as to be collected. This produces noises in the laser light source 501 and has bad influences on the laser oscillation mode. As the result, this causes the shortening of the life of the laser device, unstability (fluctuation phenomena) of the output thereof, etc. and lowers the reliability of the device for detecting foreign matter, etc. PA1 (2) Further, although the P polarized light 512b in the scattered light 512 can be detected, the S polarized light 512a cannot be detected and it is not possible to obtain any satisfactory detection sensitivity. PA1 (3) In addition, since the laser spot 511c is punctual, in order to scan the sample 1 2-dimensionally, it is necessary to dispose means for sweeping the laser beam (not shown in the figure) in the optical path within the downward illuminating optical system, which causes complication of the optical system.
Another object of the present invention is to provide a method for detecting foreign matter and a device for realizing the same capable of improving the performance for detecting foreign matter, in order to solve the problem of the prior art technique.